Surface coating castings



Patented Nov. 20, 1934 UNITED STATES PATENT OFFICE Serial No. 579,824.

Renewed June 2, 19.34

7 Claims. (Cl. 22-189) This invention relates to the surface coating of metallic castings.

Metallic molybdenum and tungsten and their carbides possess properties which adapt-them for a wide variety of uses, such as resistance to wear, certain types of corrosion, abrasion, and, particularly in the case of the carbides, for use as cutting tools and dies. Heretofore articles for such purposes have been made by compressing these materials in the form of finelyv divided powders, to shape the desired article, which is then sintered or cemented together at high temperature in an appropriate atmosphere. Other substances, such as powdered cobalt, are frequently added to act as binding or toughening agents. The price of such materials has been very high for which reason the common practice has been to join formed pieces to a cheaper base, as by welding or brazing small tips to steel bar to make cutting tools.

For many purposes it would be satisfactory to have articles surface coated with suchmaterials, instead of consisting of them throughout. Prior to my invention, however, the only means available to this end has been by welding tungsten or tungsten carbide to a metallic base. Generally it has been necessary to use electric arc welding for this purpose, and to maintain a reducing atmosphere during welding. The coatings produced in this manner have been quite limited in thickness, and their production over extended areas is slow and tedious. For these reasons this procedure has been of rather limited applicability, being restricted largely to surfacing small articles, such as tool bits and the like. No other means has been available commercially for coating metallic articles with these materials.

It is among the objects of this invention to provide metallic castings surfaced with metallic molybdenum or tungsten, their mixtures, alloys, carbides, or carbide-forming mixtures, in which the coating is adherently bonded to the base, which may be made by a simple and cheap procedure, and in the production of which extended areas may be coated and thick coatings produced.

This invention is predicated upon my discovery that its stated objects may be attained by casting molten base metal into contact with a layer of the desired coating material in finely powdered form applied to the surface of a mould of suitable shape. I have found that, contrary to what might be expected, the heat of the molten metal and/or the molten base itself by combining with it causes the coating material to unite cement or sinter and form a coating which combines with and adherently surfaces the resultant cast arused, or mixtures of tungsten and free carbon in excess of that needed to form the desired carbide. Inall cases the coating material is used in finely powdered form.

In the practice of the invention the coating material is mixed with an adhesive binder adapted to hold it in position on the mould surface,

such as the binders known and used in moulding practice. For instance, aqueous solutions of sodium silicate may be used, or glutene water and the like. The binder is mixed to suitable consistency with the metallic powder, and the mould is then surfaced with it, as by trowelling or spraying. l

The mould is dried if necessary, and molten base metal is poured into it to form the casting. Under the heat of the metal the coating material becomes cemented together to form a solid metallic, material, and likewise becomes bonded to 86 the underlying base metal, or casting. This action is contrary to what would have been expected, for these coating materials possess very high melting points, and it has been thought that extremely high temperatures and carefully controlled atmospheres are necessary for their successful manipulation.

The thickness of the layer of composition applied to the mould, and hence of the surface coating, is governed largely by the use for which the cast article is intended. The thickness of coating that can be formed on the castings'depends upon the temperature of pouring, the freezing point of. the base metal, and the thickness of the section to be coated. Using ordinary steel as base metal, and normal casting temperatures, with a composition of tungsten powder mixed with 7 to 8 per cent of carbon (i. e. an excess over that theoretically necessary to form WC), a coating 0.125 inch thick can be applied to a section 1 or 1.5 inch in thickness. Under the same conditions a coating 0.25 inch thick can be formed on a 2 inch section.

The properties of the coatings may be modifled by suitably varying the compositions used, for "0 instance y the use of mixtures as described hereinabo or by adding other metallic alloying materials. For example, the thickness of the coating may be increased by adding to the compositions about 10 percent of relatively low melting point metal or alloy, such as spiegeleisen, nickel-chromium alloys, and the like. Or, metals and alloys may be added to act as a toughening matrix for the coating material itself. In these and other cases such additions are ancillary to the main coating material. These added materials are also used in the form of fine powders, and in the practice of the invention the finer the compositions are powdered the more satisfactory the results are, in general.

The base metal may, and frequently will, combine, or alloy with, the coating composition. Therefore, it is usually desirable to use compositions containing a high percentage of the metallic coating material, e. g. commercially pure tungsten or its carbides if a high content of such material is needed in the facing. However, in some instances the composition used to surface the mould may contain as low as 20 percent of such coating elements, the balance being iron or other alloying material.

Thus the invention provides a simple and economical means for providing tools and other articles with hard or abrasion-resistant surfaces. The procedure is simpler and much more economical than those available heretofore, since the article is formed and coated in a single operation, 1. e. casting. The invention further provides forvarying the properties of the facing readily, by modifying the composition of the carbide-forming or other compositions used, or by adding alloying elements to control toughness, and in other ways. Furthermore, by suitable selection of the base metal, and heat treatment of the casting, the properties of the base may be modified. These factors combined to provide flexibility of application, to provide combinations of properties best suited to meet service requirements.

According to the provisions of the patent statutes, I have explained the principle of my inventon and have described what I now consider to represent its best embodiment. However, I desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described;

I claim:

1. A method of making base metal castings provided with a hard surface coating, comprising providing a mold with a coating comprising finely powdered material of the group consisting of molybdenum carbide, tungsten carbide, and mixtures containing molybdenum or tungsten with carbon adapted to form a carbide thereof, and pouring molten base metal into the coated mold to form the casting and thereby forming on the casting a united and adherently bonded hard coating of said carbide.

2. A method of making base metal castings provided with a hard surface coating, comprising providing a mold with a coating comprising finely powdered material of the group consisting of molybdenum carbide, tungsten carbide, and

mixtures containing molybdenum or tungsten with carbon in excess of that-required to form carbide, thereof, and pouring molten base metal into the coated mold to form the casting, and thereby forming on the casting a united and adherently bonded hard coating of said carbide.

than the materials of said group, and pouring molten base metal into the coated mold to form a the casting,and thereby forming on the castinga sintered adherently bonded hard coating of said carbide.

4. A-method according to claim 2, said base metal comprising a ferrous metal.

5. A method according to claim 3, :said base metal comprising a ferrous metal.

- 6. A composition for coating the surface of molds, comprising finely powdered material .of

the group consisting of molybdenum carbide,

tungsten carbide and mixtures containing molybdenum or tungsten with carbon in excess of that needed to form carbide thereof, and an adhesive temporary binder to hold said material in place on a mold surface, said composition under the influence of the heat of base metal poured in a mold surfaced therewith'causing it to form a united carbide coating adherently bonded to the resultant casting.

7. A composition for coating the surface of I molds, comprising finely powdered material of the group consisting of molybdenum carbide, tungsten carbide, and mixtures containing molybdenum or tungsten with carbon in excess of that needed to form carbide thereof, and also including a finely powdered metal more readily fusible than said carbides, and a temporary adhesive binder to hold the composition in place on a mold, the content of said carbide material being in excess of about 20 percent of the composition, and molten base metal poured into a mold surfaced therewith causing the composition to form a united carbide coating adherently bonded to the resultant casting.

JOSEPH W. WEI'I'ZENKORN. 

